[SunRescue] Proper use of VME scsi controllers?????

[Peter Koch]

Hi Bob!

Sun is often very conservative with such configurations.
The reason is simple: A "wild" configuration of several
disk/tape controllers with lots of hard disks introduces
flakyness!

Example: I had a 3/260 with two Sun2-SCSI-controllers
(one internally: QIC-tape and two MFM disks on ACB-4000)
and one externally (a 3GB Seagate and an Exabyte) and
a Ciprico Rimfire (two 1GB Seagate). This worked.
When i plugged in an ALM-2 to test it, the machine
started to fail every so other. It maybe ran for days
or only hours and crashed. This didn't change when i
took out the ALM. Later i changed the whole configuration
and luckily the machine became stable again.
I never found out what was the real problem.

My impression is: You can max out a single controller,
but if you have more than one, it may (and will) fail.

The Sun3 only has SCSI-I. This means you can have at
most 12 meter cable (every piece inside a case counts!).
This is enough for connecting four disks, two tapes
and a cdrom. This would be the maximum configuration
of seven devices on the SCSI bus.
Termination is a real problem on the Sun3. I found
some configurations work WITHOUT terminator that failed
WITH terminator. Funny!

Important: ALL VME-Bus SCSI controllers are TERMINATED.
This severely limits the possibility to hung internal
AND external devices to the bus.

And yes, one can hack the kernel to support more than
two SCSI controllers. I did that once and made three
Sun3-SCSI-controllers work. Another hack was to make
two Sun2-SCSI-controllers work. The GENERIC kernel
only supports one Sun2 or Sun3 and a second Sun3
controller. If you have a FEH, you can easily guess
which jumpers to set.
I never had enough SCSI controllers to test these
configurations for reliability (e.g. let them run for
several days). I doubt they will be very reliable.

>From my experience, more than 4 devices on the Sun
VME SCSI bus do not work properly. I try to minimize
as much as possible, because this avoids hassle.

If you need a lot of disk space, Seagate still sells
the 3 Gig monsters (5.25") ST43400N. The price is
very low now. They work fine on Sun3. You can even 
boot a Sun 3/50 with old PROMS or a 3/260 with a
Sun2-SCSI-controller, because this disk can disable
parity. In germany i saw them for DM 145 last month
(less than $80). I haver four of them and that's 
more than enough for my Zoo.

Once i plugged four CDROM drives to the SCSI bus.
To make this work one has to hack the kernel (but
it is very straightforward). I later learned, why
Sun had avoided to do this: The sr driver hangs
if it probes certain tape drives (my Exabytes are
an example) and so the machine doesn't come up.
A temp workaround was to switch off the Exabyte
until the kernel was loaded.

Ok, ok, i see! I'll diff my kernel configs agains
GENERIC and write an article...

Tschuess

Peter